The present application is based on and claims priority from Japanese Patent Application 2001-122909, filed Apr. 20, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a generator-motor for a vehicle.
2. Description of the Related Art
Recently, a proposal to substitute a generator-motor for a starter motor has been presented, because such a generator-motor functions not only as a starter motor but also as a generator. In addition, it can recover kinetic energy while a vehicle is braking and also assist the engine with torque while the vehicle is accelerated.
An interior permanent magnet type synchronous machine is well known as a motor generator that is simple in structure and generates a large torque at a high efficiency. Such an interior permanent magnet type synchronous machine is comprised of a rotor having a rotor core and a plurality of permanent magnets and a stator having a multi-phase armature winding. The rotor core has a plurality of auxiliary pole portions disposed between the permanent magnets. Each auxiliary pole has a circumferential width that is about the same as a circumferential width of the tooth of the stator core in order to increase the average output torque. In other words, the ratio of the circumferential width of the auxiliary pole portion to the circumferential width of the tooth is larger than 0.85.
However, the output torque of such an interior permanent magnet type synchronous machine changes with the starting position of the rotor relative to the stator thereof when it is operated as a motor. Therefore, the size of the synchronous machine has to be large enough to start the engine even if the rotor stops at a position where the output torque is at a minimum. Thus, it is difficult to use a small-sized interior permanent magnet type synchronous machine for a starter.
Therefore, the present invention has been made in view of the above problems.
It is a main object of the invention to provide a compact generator-motor that can be used as an engine starter motor.
Another object of the invention is to provide an improved compact interior permanent magnet type synchronous machine whose minimum torque curve is comparatively high and flat.
According to a feature of the invention, a generator motor is comprised of a stator having teeth and slots and a rotor having a rotor core and permanent magnets. The rotor core has a plurality of axially extending through-holes to accommodate the permanent magnets and a plurality of auxiliary pole portions disposed between the through holes. Each auxiliary pole has a circumferential width that is 30%-55% of a circumferential width of the tooth. It is preferable that each auxiliary pole has a circumferential width that is 38%-50% of a circumferential width of each tooth.